A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The liquid may be distilled water, although another liquid could be used. The description herein references a liquid. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with a higher refractive index than air, desirably a higher refractive index than water. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate, or substrate and substrate table, in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852) means that there is a large body of liquid that must be accelerated during a scanning exposure. This may mean that additional or more powerful motors are required to move the substrate and/or substrate table. Furthermore, turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system PL and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). A possible implementation of this solution is disclosed in PCT patent application publication no. WO 99/49504. As illustrated in FIGS. 2 and 3 herein, liquid is supplied by at least one inlet IN onto the substrate W, preferably along the direction of movement of the substrate relative to the final element PL, and is removed by at least one outlet OUT after having passed under the projection system PL. That is, as the substrate W is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of inlets and outlets positioned around the final element are possible; one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
A problem encountered with immersion lithographic machines is the occurrence of contaminating particles within the immersion system and on the surface of the substrate. The presence of a particle in the immersion system may cause a defect to occur during the exposure process when the particle is present between the projection system and the substrate being exposed. The same problem may be encountered if bubbles form in the immersion fluid and/or on the surface of, for example, resist provided on the substrate.